Liposome compositions and methods for the treatment of atherosclerosis

ABSTRACT

The present invention provides compositions and methods for treating atherosclerosis. The compositions comprise unilamellar liposomes having an average diameter of 100-150 nanometers. Methods for treating atherosclerosis employing the compositions of the present invention are also provided.

This is a continuation of U.S. Ser. No. 09/175,553 filed Oct. 20, 1998,now U.S. 6,139,871 which is a continuation of U.S. Ser. No. 08/507,170filed Jul. 26, 1995, now abandoned, which is a continuation of U.S. Ser.No. 08/206,415 filed Mar. 4, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The present invention provides pharmaceutical compositions and methodsuseful for the treatment of atherosclerosis. More particularly, thecompositions generally comprise liposomes having an average diameter ofabout 100-150 nanometers and a pharmaceutically acceptable carrier. Themethods generally comprise administering such compositions.

Atherosclerosis is the leading cause of death in the United States.Atherosclerosis is the formation of plaques in arterial walls that canocclude the vessel lumen and obstruct blood flow through the vessel.Morbidity and mortality generally occur through end organ damage andorgan dysfunction resulting from ischemia. The most common forms ofischemic end organ damage are myocardial infarction and cerebrovascularaccidents. Disability or death often result from these vascular events.Even atherosclerosis-related ischemia that does not permanently injuremyocardium is responsible for significant morbidity in the form ofangina pectoris and congestive heart failure. Other organs, such as thekidneys, the intestines, and the spinal cord, may also be injured byatherosclerotic occlusions. Further, in diseases such as aorticaneurysms, atherosclerotic arteries may cause clinical symptomsindependent of end organ dysfunction.

Arteriosclerotic lesions are plaques that form by accumulation ofcholesterol, cholesterol esters, and phospholipids and proliferation ofsmooth muscle cells in the intima of major arteries. Lipid contributes amajor portion of the plaque volume (generally 30-65% dry weight). Small,Arteriosclerosis, 8:103-129 (1988). In fact, the risk of developingarteriosclerosis is directly related to the concentration of certainforms of plasma cholesterol.

Lipids, including cholesterol, are generally insoluble in aqueousplasma. Plasma lipids are carried by soluble lipoprotein complexes.These lipoprotein complexes consist of an inner core of non-polar lipids(cholesteryl esters and triglycerides) and an surface layer ofhydrophilic proteins and polar lipids (phospholipids and non-esterifiedcholesterol). Different proteins are present in the surface coat ofdifferent lipoprotein complexes (lipoproteins). The differentlipoproteins perform different functions in lipid metabolism.

Five classes of lipoproteins are known. Some lipoproteins carrytriglycerides and cholesterol from the liver to peripheral tissues whileothers transport lipids to the liver. Cholesterol may be metabolized inthe liver to bile salts that are excreted, thus lowering total bodycholesterol. Two lipoproteins, low density lipoproteins (LDL) and highdensity lipoproteins (HDL), have a high degree of association with thedevelopment of atherosclerosis. LDL has a high cholesterolconcentration, delivers lipids to cells of peripheral tissues, and isassociated with a high risk of atherosclerosis. HDL also has arelatively high cholesterol concentration, but carries lipids to theliver for metabolism into bile salts and is associated with decreasingthe risk of developing atherosclerosis.

Cholesterol metabolism and homeostasis is the result of a complexequilibrium between free sterol in the cell and in plasma. Phillips etal., Biochim. Biophys. Acta, 906:223-276 (1987). Delivery of cholesterolto cells occurs via the receptor-mediated LDL pathway and by passiveexchange of sterol between plasma membranes and lipoproteins. Onlytissues that produce steroid hormones and bile acids can metabolizecholesterol. In order to prevent accumulation of excess free sterol inremaining peripheral tissues there is a reverse transport of cholesterolfrom plasma membranes into HDL and lipoprotein-like particles. HDLtransports excess cholesterol to the liver where it can either beprocessed into bile salts for excretion or incorporated into very lowdensity lipoproteins (VLDL) to re-enter the lipoprotein pool.

The passive exchange of cholesterol between cells and lipoproteinsoccurs via the diffusion of sterol molecules across the aqueous space.Phillips et al., supra, and Schroeder et al., Exp. Biol. Med.,196:235-252 (1991). Net cellular efflux occurs if the chemical potentialof free cholesterol is lower in the plasma than in the cells so thatsterol leaves the membrane following its activity gradient. Under theseconditions, it has been shown that cholesterol-ester-loaded cells, whichare morphologically characteristic of early atherosclerotic lesions, notonly lose cholesterol, but promote ester hydrolysis, resulting in thereduction of intracellular deposits of this lipid. Small,Arteriosclerosis, 8:103-129 (1988). Moreover as mentioned above, thereis epidemiological evidence that conditions which might be expected toenhance reverse cholesterol transport (low plasma cholesterolconcentrations, or increased HDL concentrations) are correlated withreduced risk of premature atherosclerosis and may give rise to plaqueregression.

Characteristically, plaques are associated with ulceration of the vesselintima. The lipid-containing plaques grow in the ulcerations projectingfriable masses into the arterial lumen. The plaques may also injure andweaken the smooth muscle media of the vessel. As plaque formationprogresses, more central regions of the plaques are shielded from thecirculation. Extensive plaque formation also cause concentricconstriction of the vessel at the plaque site.

Presently, the most effective treatment of atherosclerosis isprevention. There is evidence that the progression and accumulation oflipids in lesions can be halted when plasma LDL concentrations are keptto near normal levels. Reynolds, Circulation, 79:1146-1148 (1989).Current preventive management of atherosclerotic disease has focused onthe use of drugs in conjunction with dietary restrictions to regulateplasma cholesterol levels. Moreover, antioxidant therapies whichsuppress the formation and uptake of modified LDL particles by the cellsof the arterial wall are also proving beneficial. Chisolm, Clin.Cardiol., 14:25-30 (1991). However, while hypocholesterolemic drugsinduce favorable plasma cholesterol changes which appear to slow theprogression of atherosclerosis, they do not generally induce conditionsthat promote the efflux and removal of atheroma cholesterol. Clearly, inorder to achieve significant regression of atheroma and lessen lumenobstruction, these space occupying lipids must be mobilized. Presentevidence suggests that processes which stimulate the efflux ofextrahepatic cell cholesterol and transport it to the liver forexcretion, reverse cholesterol transport (RCT), are important events inthe prevention of atherosclerosis. Gwynne, Clin. Cardiol., 14:17-24(1991).

Current therapeutic modalities of arteriosclerosis are generally dividedinto surgical and medical management. Surgical therapy may entailvascular graft procedures to bypass regions of occlusion (e.g., coronaryartery bypass grafting), removal of occluding plaques from the arterialwall (e.g., carotid endarterectomy), or percutaneously cracking theplaques (e.g., balloon angioplasty). Surgical therapies carrysignificant risk and only treat isolated lesions. Atheroscleroticplaques downstream from the treated lesion may continue to obstructblood flow. Surgical therapies also do not limit the progression ofatherosclerosis and are associated with the late complication ofrestenosis.

Medical therapy is directed to reducing other risk factors related tovascular disease (e.g., smoking, diabetes, and hypertension) andlowering forms of serum cholesterol that are associated with thedevelopment of atherosclerosis as described above. While medicaltherapies may slow the progression of plaque formation, plaqueregression is relatively rare. Therefore, symptomatic atherosclerosisoften requires both surgical and medical treatment.

Paradoxically, intravenous infusion of phospholipids and liposomes hasbeen shown to produce regression of atherosclerotic plaques althoughserum lipid levels are transiently elevated. Williams et al., Perspect.Biol. Med., 27:417-431 (1984). In some instances, however, cholesterolassociated with development and progression of atherosclerosis mayincrease following liposome administration.

Previous studies investigating phospholipid-induced mobilization ofcholesterol in vivo have employed multilamellar or sonicated liposomevesicles. Liposome size is a key characteristic in clearance kineticsand is one of several reasons why sonicated vesicles have been expectedto represent the bilayer structure best suited to enhance reversecholesterol transport. Sonication reduces multilamellar vesicles (MLV)to ‘limit size’ vesicles. These systems exhibit the minimum radius ofcurvature that can be adopted by the bilayer configuration withoutdisruption. For example, the minimum size egg phosphatidylcholineliposome that can be generated is typically about 30-nm diameter, oftenclassified as a small unilamellar vesicle (SUV). For a given liposomecomposition, it is generally assumed that the smaller the particlediameter the greater the circulation half-life (Gregoriadis and Senior,Life Sci., 113:183-192 (1986)). Consequently, it was expected that SUVcomposed of phosphatidylcholine would circulate longer than largerliposomes, and therefore mobilize more cholesterol. Furthermore, packingconstraints experienced by phospholipids in SUV, (due to the acuteradius of curvature) gives rise to an instability that can result infusion, Hope et al., Chem. Phys. Lipids, 40:89-107 (1986), as well as anincreased tendency to assimilate with lipoproteins. See, e.g., Scherphofet al., Biochim. Biophys. Acta, 542:296-307 (1978) and Krupp et al.,Biochim. Biophys. Acta, 72:1251-1258 (1976). Therefore, it was expectedthat SUV would produce a greater number of HDL-like particles, thuspromoting efflux of sterol from peripheral tissues. Supporting thisexpectation, liposomes having diameters of 50-80 nm have been reportedto optimize sterol mobilization and plaque regression. European PatentPublication No. 0461559A2.

What is needed in the art is a medical treatment for atherosclerosisthat not only will slow progression of lesions, but also predictablycause regression and shrinkage of established plaques. Such a treatmentshould provide the optimal rate of cholesterol removal (and, henceshrinkage) from plaques. Quite surprisingly, the present inventionfulfills these and other related needs.

SUMMARY OF THE INVENTION

The present invention provides pharmaceutical compositions consistingessentially of unilamellar liposomes having an average diameter of about100-150 nanometers, which liposomes are not bound to a drug; and apharmaceutically acceptable carrier. These liposomes optimizecholesterol efflux from atherosclerotic plaques. The liposomes may bebound to an apoprotein, typically apoprotein A1 or A2. The liposomesoften contain at least one phospholipid, such as phosphatidylcholine orphosphatidylglycerol. Liposomes having diameters of about 125 nm arepreferred.

Also provided are methods for treating atherosclerosis employing thepharmaceutical compositions of the present invention. The compositionsare administered to animals having atherosclerosis. Often, thecompositions will be serially administered over a period of time.Generally, the compositions will be administered parenterally, usuallyintravenously. The methods may be employed therapeutically orprophylactically. The methods of the present invention are also usefulfor treatment of hypoalphalipoproteinemia and hyperlipidemias.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B demonstrate cholesterol mobilization by a homogeneouspopulation of large unilamellar vesicles with a mean diameter of 125 nm.

FIGS. 2A and 2B illustrate plasma cholesterol distribution in normal andliposome-treated animals.

FIG. 3 illustrates liposome cholesterol accumulation over a 24-htime-course in vivo.

FIGS. 4A and 4B illustrate cholesterol mobilization by liposomes.

FIG. 5 illustrates a comparison of the rate of cholesterol accumulationby unilamellar and oligolamellar liposomes.

FIGS. 6A and 6B demonstrate the cholesterol mobilizing ability ofliposomes having different compositions.

FIGS. 7A, 7B and 7C illustrate the cholesterol content of erythrocytesin mice treated with liposomes and untreated mice.

FIGS. 8A and 8B illustrate plasma cholesterol concentration changes inrabbits treated with liposomes and untreated rabbits.

FIG. 9 illustrates plasma phospholipid concentration changes in rabbitstreated with liposomes and untreated rabbits.

FIG. 10 demonstrates the quantity of cholesterol mobilized by liposomesduring treatment of rabbits.

FIGS. 11A and 11B illustrates clearance profiles of liposomes injectedinto rabbits.

FIG. 12 demonstrates the cholesterol:phospholipid ratio of lipoproteinsfollowing liposome injection.

FIGS. 13A, 13B and 13C illustrates aortic cholesterol content inliposome and saline treated rabbits.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides pharmaceutical compositions consistingessentially of unilamellar liposomes having an average diameter of about100-150 nanometers, which liposomes are not bound to a drug; and apharmaceutically acceptable carrier. Also provided are methods fortreating atherosclerosis using the compositions of the presentinvention.

As used herein, “drug” is meant to indicate a synthetic compoundsuitable for therapeutic use without associated bound carriers,adjuvants, activators, or co-factors. “Drug” does not includeapoproteins, lecithin-cholesterol acyltransferase, or albumin.“Liposome”, “vesicle” and “liposome vesicle” will be understood toindicate structures having lipid-containing membranes enclosing anaqueous interior. The structures may have or one more lipid membranesunless otherwise indicated, although generally the liposomes will haveonly one membrane. Such single layered liposomes are referred to hereinas “unilamellar”.

Arterial atherosclerotic lesions have been shown to regress when treatedwith liposome infusions. In some instances, however, LDL cholesterol hasbeen observed to increase following liposome administration. Prior tothe present invention, the cause of this paradox has not beenunderstood.

The present invention is based, in part, on the discovery that liposomesize plays a critical role in the metabolism of cholesterol removed fromatherosclerotic plaques by the liposomes. Contrary to previousdescriptions of liposome therapy, liposomes having a diameter of greaterthan 100 nanometers are more effective for removing cholesterol fromatherosclerotic plaques than smaller liposomes.

The superior action of liposomes greater than 100 nanometers in diametermay be explained by the micro-anatomy of the liver. When circulating inthe liver, large liposomes (as used herein, liposomes greater than 100nm in diameter) may be cleared by the Kupffer cells that line thesinusoidal openings. The Kupffer cells transfer cholesterol tohepatocytes for excretion in the bile or re-utilization. Small liposomes(as used herein, liposomes smaller than 100 nm) may directly accesshepatocytes without prior processing by the Kupffer cells. Because thesesmall liposomes are infused in relatively large doses, hepatocytes maybe acutely exposed to a relatively high concentration of small liposomesand their accumulated cholesterol.

The pharmaceutical compositions of the present invention generallyconsist essentially of unilamellar liposomes having an average diameterof about 100-150 nanometers, which liposomes are not bound to a drug;and a pharmaceutically acceptable carrier. In some instancesmultilamellar liposomes may also be employed in the compositions of thepresent invention, either exclusively or in addition to unilamellarliposomes. The liposomes will have an average diameter of about 100-150nanometers, typically about 125-140 nanometers. In some embodiments,liposomes having an average diameter larger than 150 nanometers, eitheruni- or multilamellar, may also be present in the compositions of thepresent invention.

Persons of skill will appreciate that the liposomes in the compositionsof the present invention may be synthesized by a variety of methods,such as described in, e.g., U.S. Pat. Nos. 4,186,183; 4,217,344;4,261,975; 4,485,054; 4,774,085; 4,946,787; PCT Publication No. WO91/17424, Deamer and Bangham, Biochim. Biophys. Acta, 443:629-634(1976); Fraley et al., Proc. Natl. Acad. Sci. USA, 76:3348-3352 (1979);Hope et al., Biochim. Biophys. Acta, 812:55-65 (1985); Mayer et al.,Biochim. Biophys. Acta, 858:161-168 (1986); and Williams et al., Proc.Natl. Acad. Sci., 85:242-246 (1988), each of which is incorporatedherein by reference. Suitable methods include, e.g., sonication,extrusion, high pressure/homogenization, microfluidization, detergentdialysis, calcium-induced fusion of small liposome vesicles, andether-infusion methods, all well known in the art.

Generally, the liposomes are most conveniently generated by sonicationand extrusion procedures. Briefly, a chloroform solution of lipid isvortexed and the solvent removed under a steady stream of N₂. The sampleis dried under a high vacuum. The resulting dry lipid film is rehydratedin 150 mM NaCl and 20 mM [4-(2-hydroxyethyl)]-piperazine-ethanesulfonicacid (Hepes, pH 7.4). This generally produces multilamellar liposomalvesicles. Unilamellar vesicles are prepared by sonication or extrusion.

Sonication is generally performed with a tip sonifier, such as a Bransontip sonifier, in an ice bath. Typically, the suspension is subjected toseveral sonication cycles. Extrusion may be carried out by biomembraneextruders, such as the Lipex Biomembrane Extruder. Defined pore size inthe extrusion filters may generate unilamellar liposomal vesicles ofspecific sizes. The liposomes may also be formed by extrusion through anasymmetric ceramic filter, such as a Ceraflow Microfilter, commerciallyavailable from the Norton Company, Worcester Mass.

The size of the liposomal vesicles may be determined by quasi-electriclight scattering (QELS) as described in Bloomfield, Ann. Rev. Biophys.Bioeng., 10:421-450 (1981), incorporated herein by reference. Averageliposome diameter may be reduced by sonication of formed liposomes.Intermittent sonication cycles may be alternated with QELS assessment toguide efficient liposome synthesis. The liposomes may be composed of avariety of lipids. Generally, the liposomes will be composed of at leastone phospholipid, typically egg phosphatidylcholine, eggphosphatidylglycerol, distearoylphosphatidylcholine, ordistearoylphosphatidylglycerol. Many embodiments of the presentinvention will include more than one phospholipid.

Other phospholipids suitable for formation of liposomes comprising thecompositions of the present invention include, e.g.,phosphatidylcholine, phosphatidylglycerol, lecithin, β,γ-dipalmitoyl-α-lecithin, sphingomyelin, phosphatidylserine,phosphatidic acid,N-(2,3-di(9-(Z)-octadecenyloxy))-prop-1-yl-N,N,N-trimethylammoniumchloride, phosphatidylethanolamine, lysolecithin,lysophosphatidylethanolamine, phosphatidylinositol, cephalin,cardiolipin, cerebrosides, dicetylphosphate,dioleoylphosphatidylcholine, dipalmitoylphosphatidylcholine,dipalmitoylphosphatidylglycerol, dioleoylphosphatidylglycerol,palmitoyl-oleoyl-phosphatidylcholine, di-stearoyl-phosphatidylcholine,stearoyl-palmitoyl-phosphatidylcholine,di-palmitoyl-phosphatidylethanolamine,di-stearoyl-phosphatidylethanolamine, di-myrstoyl-phosphatidylserine,di-oleyl-phosphatidylcholine, and the like. Non-phosphorus containinglipids may also be used in the liposomes of the compositions of thepresent invention. These include, e.g., stearylamine, docecylamine,acetyl palmitate, fatty acid amides, and the like. Additional lipidssuitable for use in the liposomes of the present invention are wellknown to persons of skill in the art and are cited in a variety of wellknown sources, e.g., McCutcheon's Detergents and Emulsifiers andMcCutcheon's Functional Materials, Allured Publishing Co., Ridgewood,N.J., both of which are incorporated herein by reference.

Generally, it is desirable that the liposomes be composed of lipids thatare liquid-crystalline at 37° C., often at 35° C., and even 32° C.Liposomes in the liquid-crystalline state typically accept cholesterolmore efficiently than liposomes in the gel state. As patients typicallyhave a core temperature of about 37° C., liposomes composed of lipidsthat are liquid-crystalline at 37° C. are generally in aliquid-crystalline state during treatment and, therefore, optimizeremoval of cholesterol from plaques.

The pharmaceutical compositions of the present invention also comprise apharmaceutically acceptable carrier. Many pharmaceutically acceptablecarriers may be employed in the compositions of the present invention.Generally, normal saline will be employed as the pharmaceuticallyacceptable carrier. Other suitable carriers include, e.g., water,buffered water, 0.4% saline, 0.3% glycine, and the like, includingglycoproteins for enhanced stability, such as albumin, lipoprotein,globulin, etc. These compositions may be sterilized by conventional,well known sterilization techniques. The resulting aqueous solutions maybe packaged for use or filtered under aseptic conditions andlyophilized, the lyophilized preparation being combined with a sterileaqueous solution prior to administration. The compositions may containpharmaceutically acceptable auxiliary substances as required toapproximate physiological conditions, such as pH adjusting and bufferingagents, tonicity adjusting agents and the like, for example, sodiumacetate, sodium lactate, sodium chloride, potassium chloride, calciumchloride, etc.

The concentration of liposomes in the carrier may vary. Generally, theconcentration will be about 20-200 mg/ml, usually about 50-150 mg/ml,and most usually about 100 mg/ml. Persons of skill may vary theseconcentrations to optimize treatment with different liposomal componentsor of particular patients. For example, the concentration may beincreased to lower the fluid load associated with treatment. This may beparticularly desirable in patients having atherosclerosis-associatedcongestive heart failure or severe hypertension. Alternatively,liposomes composed of irritating lipids may be diluted to lowconcentrations to lessen inflammation at the site of administration.

The liposomes may also be bound to a variety of proteins andpolypeptides to increase the rate of cholesterol transfer or thecholesterol-carrying capacity of the liposomes. Binding of apoproteinsto the liposomes is particularly useful. As used herein, “bound toliposomes” or “binding to liposomes” indicates that the subject compoundis covalently or non-covalently bound to the surface of the liposome orcontained, wholly or partially, in the interior of the liposome.Apoprotein A₁, apoprotein A₂, and apoprotein E will generally be themost useful apoproteins to bind to the liposomes. These apoproteinspromote transfer of cholesterol and cholesteryl esters to the liver formetabolism. Lecithin-cholesterol acyltransferase is also useful formetabolizing free cholesterol to cholesteryl esters. Liposomes in thepharmaceutical compositions of the present invention may be bound tomolecules of apoprotein A₁, apoprotein A₂, and lecithin-cholesterolacyltransferase, singly or in any combination and molar ratio.Additional proteins or other non-protein molecules may also be useful tobind to the liposomes to enhance liposome stability or half-life and thelike. These include, e.g., cholesterol, polyethyleneglycol,alkylsulfates, ammonium bromide, albumin, and the like.

Also provided are methods for treating atherosclerosis in an animal. Themethods generally comprise administering a liposome composition to theanimal, which liposome composition consists essentially of unilamellarliposomes having an average diameter of about 100-150 nanometers. By“treating atherosclerosis”, it is meant performing a therapeuticintervention that results in reducing the cholesterol content of atleast one atherosclerotic plaque or prophylactically inhibiting orpreventing the formation or expansion of an atherosclerotic plaque.Generally, the volume of the atherosclerotic plaque, and hence thedegree of obstruction of the vascular lumen, will also be reduced. Thepresent methods are particularly useful for treating atheroscleroticlesions associated with familial hyperlipidemias.

The methods of the present invention may reduce the cholesterol contentof atherosclerotic plaques and/or the volume of atherosclerotic plaques.The cholesterol content will generally be reduced by at least 10%-30%,often by 30%-50%, and in some instances as much as 75%-85% or more. Thevolume of the atherosclerotic plaques will also generally be reduced.The reduction in plaque volume will generally be at least 5%-30%, oftenas much as 50%, and in some instances 75% or more.

Cholesterol may be mobilized from the plaques by either direct effluxinto the liposomes or into lipoproteins that subsequently transfer thecholesterol to the liposomes. As cholesterol is transferred to theliposomes from the lipoproteins, the lipoproteins may receive morecholesterol from plaques. Generally, when cholesterol is received fromlipoproteins, the cholesterol is transferred from HDL.

The methods may be useful to treat atherosclerosis in a variety ofanimals and in a variety of blood vessels. Typically, the animal will behuman, although non-human primates, dogs, cats, rodents, horses, cows,and the like may be treated by the methods of the present invention.Atherosclerosis of any blood vessel, such as the aorta, carotid arteries(common, internal, and external), coronary arteries, mesentericarteries, renal arteries, iliac arteries, popliteal arteries, and thelike, may also be treated by the methods of the present invention.

The methods may also be useful for prophylactic treatments. Suchprophylactic treatments are particularly useful following invasivevascular procedures. Vascular regions having injured endothelium are atincreased risk for developing atherosclerotic plaques. Therefore,invasive vascular procedures, such as coronary angioplasty, vascularbypass grafting, and other procedures that injure the vascularendothelial layer, may be practiced in conjunction with the methods ofthe present invention. As the invasive procedure injures theendothelium, the liposomes act to remove cholesterol from the injuredregion and inhibit or prevent plaque formation of expansion duringendothelial healing.

Hyperlipidemias may also be treated by the methods of the presentinvention. Administration of liposomes, alone or bound to apoprotein A₁and apoprotein A₂, to individuals having hypoalphalipoproteinemia fromgenetic or secondary causes, familial combined hyperlipidemia, andfamilial hypercholesterolemia is a useful treatment.

The liposomes administered in the methods of the present invention willbe composed of lipids as described above. The lipids will generally bein the liquid-crystalline state at 37° C. The lipids will also generallyinclude one or more phospholipids, often phosphatidylcholine orphosphatidylglycerol, although liposomes may be composed of many otherlipids, examples of which are described above.

The liposomes may be administered in many ways. These include parenteralroutes of administration, such as intravenous, intramuscular,subcutaneous, and intraarterial. Generally, the liposomes will beadministered intravenously. Often, the liposomes will be administeredinto a large central vein, such as the superior vena cava or inferiorvena cava, to allow highly concentrated solutions to be administeredinto large volume and flow vessels. The liposomes may be administeredintraarterially following vascular procedures to deliver a highconcentration directly to an affected vessel. The liposomes may also beadministered directly to vessels in a topical manner by surgeons duringopen procedures. In some instances, the liposomes may be administeredorally or transdermally. The liposomes may also be incorporated invascular stents for long duration release following placement. This isparticularly effective for angioplasty treatment of restenosis oflesions in the coronary arteries.

As described above, the liposomes will generally be administeredintravenously in the methods of the present invention. Often multipletreatments will be given to the patient, generally weekly. Typically,the therapy will continue for about 4-16 weeks (4-16 treatments),usually about 10 weeks (10 treatments). The duration and schedule oftreatments may be varied by methods well known to those of skill.

The dose of liposomes may vary depending on the clinical condition andsize of the animal or patient receiving treatment. Humans will generallybe treated with about 0.1-1.5 gm of liposomes/kg body weight, usuallyabout 0.2-0.75 gm gm/kg, and most usually about 0.28-0.42 gm/kg. Thus,an average 70 kg person would be treated with about 20-30 gms. ofliposomes per treatment. The dose will typically be constant over thecourse of treatment, although the dose may vary. Serum measurements oftotal free cholesterol, total esterified cholesterol, HDL cholesterol,LDL cholesterol, and VLDL cholesterol may be used to assess and modifydosage amounts and schedules during the treatment regimen. Ascholesterol is mobilized from plaques, total serum cholesterol rises. Itis desirable that total serum cholesterol and HDL cholesterol riseduring therapy, and esterified cholesterol drop during therapy. Theliposome dose for different animals will generally approximate the humanweight-determined dosage.

The following examples are offered by way of illustration and notlimitation.

EXAMPLES Example 1 Influence of Liposome Size and Composition on In VivoCholesterol Mobilization

This example demonstrates the relative cholesterol mobilizing efficacyof liposomes of different sizes and compositions in mice. Liposomeshaving a mean diameter of about 125 nm were found to be the mosteffective in mobilizing cholesterol in vivo. Liquid-crystallineliposomes were more effective in mobilizing cholesterol than gel-stateliposomes.

Cholesterol and [4-(2-hydroxyethyl)]piperazineethanesulfonic acid(Hepes) were obtained from Sigma. [¹⁴C]cholesterol hexadecyl ether and[³H]cholesterol were purchased from New England Nuclear. Eggphosphatidylcholine (EPC), distearoylphosphatidylcholine (DSPC),distearoylphosphatidylglycerol (DSPG) and egg phosphatidylglycerol (EPG)were supplied by Avanti Polar Lipids. Bio-Gel™ A-15m medium waspurchased from Bio-Rad. All chemicals, thin layer chromatography platesand solvents were of analytical grade and purchased from BDH Chemicals.

All liposome preparations were labelled using trace amounts of[¹⁴C]cholesterol hexadecyl ether (CHE). This labelling is useful as (1)it does not undergo passive exchange between membranes; (2) mice do notexhibit cholesterol-ester exchange protein activity; and (3) theether-linked fatty acid is not cleaved in the plasma. Consequently, inthis model system CHE is an excellent liposome marker and vesicleconcentrations in the plasma were estimated from the specific activityof this label.

A chloroform solution of EPC and [¹⁴C]CHE was vortexed and solvent wasremoved under a stream of N₂. The sample was dried under high vacuum for2 h. The dry lipid film was hydrated in 150 mM NaCl, 20 mM Hepes (pH7.4) to generate multilamellar vesicles (MLV). Vesicles were preparedfrom MLV either by sonication, to generate small unilamellar vesicles(SUV) or extrusion to produce large unilamellar vesicles (LUV).Sonication was performed using a Branson tip sonifier, followingstandard protocols. The MLV suspension was diluted to 30 mg/ml, immersedin an ice bath and subjected to 3 cycles of sonication, each of 10-minduration. The initial milky suspension became clear and the vesicle sizewas 30 nm, as determined by quasi-elastic light scattering (QELS). TheSUV were centrifuged at 10000×g for 30 min to remove titanium fragmentsoriginating from the sonicator tip.

Extrusion was carried out using a 10 ml Lipex Biomembranes Extruderequipped with a water jacketed thermobarrel as described by Hope et al.,Biochim. Biophys. Acta, 812:55-65 (1985), incorporated herein byreference. MLV were sized through two stacked polycarbonate filters ofdefined pore size to generate a variety of LUV and homogeneous MLV asdescribed in Hope et al., supra, and Mayer et al., Biochim. Biophys.Acta, 858:161-168 (1986), incorporated herein by reference.

The size of vesicles generated by sonication and extrusion procedureswas determined by QELS analysis utilizing a Nicomp Model 370 submicronlaser particle sizer equipped with a 5-mW He-Ne Laser. The Nicomp QELSanalyzes fluctuations in light-scattering intensities due to vesiclediffusion in solution. The measured diffusion coefficient is used toobtain the average hydrodynamic radius and thus, the mean diameter ofvesicles. The following diameters are expressed as the mean ±S.D. ofvesicle preparations prior to injection. Vesicles prepared by sonicationwere 30±7 nm in diameter (SUV₃₀). Vesicles prepared by extrusion throughfilters with a pore size of 0.05 μm were 70±19 μm, 0.1 μm pore size were125±30, and 0.4 μm pore size were 237±90 nm. Generally, the vesiclesprepared by extrusion are referred to herein by the filter pore sizeused in their preparation, i.e., LUV₅₀, LUV₁₀₀ and MLV₄₀₀.

Female BDF-1 or CD-1 mice, weighing 20-22 g (Sprague-Dawley), were usedthroughout this study. Liposomes were injected via the tail vein at adose of 300 mg/kg, which was typically 6 mg of liposomes in 200 ml ofbuffer injected for each animal. Control mice were injected with anequal volume of buffer and both groups were sacrificed at specifiedtimes with blood collection in EDTA microtainer tubes by heart puncture.Plasma was obtained following centrifugation at 2000×g for 10 min, andan aliquot removed for scintillation analysis using a Beckman LS 3801liquid scintillation counter. The average of data from 16 mice (fromfour separate experiments) is indicated at each time point, unlessindicated otherwise.

A 27×1.5 cm Bio-Gel™ A-15 m gel filtration column, equilibrated with 150mM NaCl, 10 mM Tris, 0.1% EDTA, 0.3% NaN₃ (pH 7.4) was used tofractionate plasma samples. Columns were eluted at a flow rate of 1ml/min and 1-ml fractions were collected for radioactivity and lipidanalyses. Data on the cholesterol:phospholipid (C/P) ratio of vesiclesand lipoproteins after infusion was obtained from pooled fractionscorresponding to the liposomal and lipoprotein peaks. The Bio-Gel™columns were calibrated with respect to lipoprotein elution by preparingpurified human lipoprotein fractions using standard ultracentrifugationprocedures as described in Schumaker et al., Methods Enzymol.,128:155-181 (1986), incorporated herein by reference. The lipoproteinfractions were each labelled with [³H]cholesterol. The elution profilesof the columns were monitored for radioactivity.

Pooled column fractions and plasma samples were extracted employing theBligh and Dyer procedure. Bligh and Dyer, Can. J. Biochem. Physiol.,37:911-917 (1959), incorporated herein by reference. The lipid extractswere analyzed for total cholesterol using the assay method of Rudell andMorris, J. Lipid Res., 14:364-366 (1973). Free and esterifiedcholesterol concentrations were determined following separation by TLCusing hexane/ether/acetic acid (70:30:1 (v/v)). Standards were used toidentify the area of the plate corresponding to these two lipids, thesilica was aspirated and the lipid eluted for assay usingchloroform/methanol (2:1 (v/v)). Plasma vesicle phospholipid content wasdetermined by dividing [¹⁴C]CHE radioactivity by liposome-specificactivity and phospholipid concentrations were determined by the methodof Fiske and SubbaRow, J. Biol. Chem., 66:375-400 (1925). Erythrocyteswere extracted using the method of Rose and Oklander (J. Lipid Res.,6:428-431 (1965)), followed by a Bligh and Dyer wash to remove residualsalts. An aliquot of red blood cells was retained for cell numberdetermination using a Coulter cell counter in order to expresscholesterol and phospholipid concentrations as mmol/10⁹ cells.

Blood was pooled from a group of mice and red cells packed by low-speedcentrifugation. The serum was labelled with [³H]cholesterol byincubation for 10 min at 37° C. with 100 μCi of radioisotope dried fromethanol. The labelled serum was added to the packed cells and themixture incubated at room temperature for 30 min. The cells were washedand approximately 10⁶ dpm of [³H]cholesterol-labelled cells injectedinto the experimental groups via the tail vein. Approximately 1 minafter the injection of cells, saline or liposomes were administered.

Donor and acceptor liposomes were separated employing ion exchangechromatography. A 10-fold excess of donor vesicles (100 nm diameter)composed of EPC/EPG/Chol (40:15:45 molar ratio) were incubated with100-nm or 400-mm EPC acceptors. Donor liposomes were labelled with[³H]cholesterol at 5 μCi/100 mg total lipid and acceptors were labelledwith [¹⁴C]CHE at 0.5 μCi/100 mg lipid. At specified time intervals, 50μl aliquots of the incubation mixture (1 mg acceptor+10 mg donor/ml)were removed and passed down a DEAE-Sepharose™ 6B-CL column prepared ina 1-ml tuberculin syringe equilibrated with 30 mM NaCl, 20 mM Hepes (pH8.0). Columns were spun at 1000×g for 1 min prior to applying aliquotsof the incubation mixture. The liposome mixture was spun through thecolumn and the eluant (acceptors) obtained with two subsequent wash/spincycles with 500-ml aliquots of buffer. Recovery of ¹⁴C-labelled vesicles(acceptors) was typically >90%. Control experiments in which donors werelabelled with a non-exchangeable marker indicated that all of the donorvesicles bound to the ion exchange column under the conditions of theexperiment. Cholesterol accumulation by acceptors was determined usingan LS 3801 Beckman scintillation counter equipped with a ¹⁴C/³Hdual-label program.

Two groups of mice (n=4) were maintained in metabolic cages and faecescollected daily. After 3 days one group was injected with 200 μl ofsaline and the second group with approx. 6 mg of EPC LUV₁₀₀ (dose 300mg/kg). Faecal material was collected for a further 7 days. Samples wereextracted using an isopropanol/chloroform extraction procedure andsubsequently assayed for total cholesterol, free cholesterol andcholesteryl esters, as described above.

Experiments were carried out on mice maintained on regular, laboratoryfood for rodents (cholesterol excretion rate 10-12 μmol/g faeces) and onTeklad low cholesterol casein-based diet which resulted in an excretionrate of approx. 0.8 μmol cholesterol/g faeces).

FIG. 1 demonstrates cholesterol mobilization by a homogeneous populationof LUV with a mean diameter of 125 nm as determined by QELS (referred toas LUV₁₀₀ and prepared by extrusion as described above). A dramaticincrease in plasma cholesterol was observed for animals receivingliposomes (FIG. 1A (circles)). Sterol levels peaked 4-8 h afterinjection at a concentration nearly double that measured in control miceinjected with an equivalent volume of saline (squares). Plasmacholesterol concentrations gradually returned to normal levels after 48h correlating well with the liposome clearance profile shown in FIG. 1B.Liposomes were labelled with trace amounts of [¹⁴C]CHE, anon-exchangeable, non-metabolizable marker frequently used to monitorliposome clearance and distribution in vivo.

Using gel filtration as described above, mouse plasma was fractionatedand the cholesterol profile determined using the chemical assayprocedure of Rudel and Morris. Plasma from control and liposome-treatedanimals were compared and the results are shown in FIGS. 2A and 2B. FIG.2A shows a normal cholesterol distribution with the majority ofcholesterol associated with combined LDL and HDL peaks (fractions22-50). The elution volumes of VLDL, LDL and HDL were determined asdescribed above. A minor quantity of sterol was detected in the voidvolume, corresponding to the larger chylomicron and VLDL lipoproteinparticles, but quantitatively these fractions represent >5% of the totalcholesterol content of the plasma. The elution profile of plasma fromliposome-treated animals (4 h time point) is shown in FIG. 2B. The[¹⁴C]CHE liposome marker was almost exclusively detected in the voidvolume, indicating that the LUV₁₀₀ were well separated from thefractions containing LDL and HDL (liposomes smaller than 100-nm diameterare included in the gel and cannot be separated from LDL). The absenceof radioactivity in the remaining fractions indicated that little, ifany, assimilation of vesicles into the lipoprotein pool occurred.However, it is possible that small quantities of vesicles had undergonestructural transitions to lipoprotein-like particles, but were removedrapidly from the circulation and therefore, not detected.

The cholesterol content of column fractions shown in FIG. 2B clearlyshows that the excess sterol in the plasma of treated mice is associatedwith LUV. The slight frame shift of peaks between FIG. 2A and FIG. 2B isthe result of differences in elution rate and not due to changes inlipoprotein size. Using TLC analysis it was determined that >90% of theliposomal cholesterol was free cholesterol, the remainder beingcholesterol ester.

The excellent separation of LUV₁₀₀ from the quantitatively most abundantlipoproteins enabled straight-forward isolation and subsequent analysisof the vesicle lipids. Liposome cholesterol accumulation was shown bythe increasing C/P ratio of vesicles over a 24-h time-course in vivo, asshown in FIG. 3. Consequently, after 24 h the liposomes remaining in thecirculation (approx. 10-15% of the initial dose) were in equilibriumwith respect to cholesterol and net sterol movement was negligible.

Plasma cholesterol concentrations were measured over a 48-h period inanimals treated with a variety of liposomal preparations varying indiameter from 30-250 nm. Sonicated vesicles were prepared as describedabove. The remaining vesicles were produced by extrusion of MLV throughfilters with defined pore-sizes to give vesicle populations with themean diameters described above. Vesicles are referred to by the filterpore size used for their synthesis.

The amount of cholesterol accumulated and removed by liposomes in vivois a function of both the rate of cholesterol uptake and the rate ofliposome clearance. An estimate of the mass of cholesterol removed fromthe circulation (mostly by the RES) can be made by calculating the C/Pratio of vesicles in vivo from plasma concentration of vesiclephospholipid and cholesterol as the excess plasma concentration abovethe control at the various experimental time points. All cholesterolabove control levels is associated with circulating liposomes. Theplasma volume of mice used in these studies was approx. 1 ml,consequently the total amount of phospholipid cleared from thecirculation between time points was known. Using the average C/P ratiomeasured for vesicles between each assay interval an estimate of theamount of cholesterol removed was obtained. The analysis was notcontinued beyond the point where less than 5% of the initialphospholipid dose remained in the circulation as below this level themeasurement error was too large to determine accurate C/P ratios. FIG.4A shows the cumulative level of cholesterol (stars) removed by LUV₁₀₀up to the time when approx. 5% of the dose remains. After 40 h 2800 nmolof cholesterol were removed from the circulation by the RES, whichrepresents 33 mol% of the injected phospholipid dose. This analysis wasused to compare the various liposomal preparations tested. For eachpreparation the plasma cholesterol (stars) and phospholipid clearanceprofiles were determined and analyzed as described above. The results inFIG. 4B show that LUV mobilize cholesterol most efficiently.

The transfer of sterol from donor vesicles to unilamellar andmultilamellar vesicles was studied. Using freeze-fracture electronmicroscopy and NMR analysis, it has been shown that MLV sized through400-nm pores retain a number of internal lamellae and therefore cannotbe classified as LUV. The transbilayer movement (flip-flop) ofcholesterol is rapid, on the order of seconds to minutes in a liquidcrystalline bilayer under conditions that promote net sterol flux.Consequently, it was expected that multilamellar systems would act as agood sink for cholesterol as sterol should rapidly disperse through theinternal lamellae.

Using an in vitro model in which LUV₁₀₀ or MLV₄₀₀ were incubated with a10-fold excess of donor liposomes containing tritiated cholesterol asdescribed above, the net transfer of sterol from donor to acceptor wasmonitored. The rate of cholesterol accumulation in the unilamellarpreparation was greater than that observed for the oligolamellarvesicles. It is interesting to note that in the presence of a 10-foldexcess of donor vesicles the equilibrium C/P ratio of the acceptorshould be approx. 0.9:1. The data in FIG. 5 show that the 100-nmacceptors only achieve a ratio of 0.35:1 after 8 h at 37° C. This isapproximately half the rate of accumulation observed for the samevesicles in vivo (FIG. 3).

The cholesterol mobilizing properties of two types of LUV₁₀₀ werecompared. The two types of LUV₁₀₀ were composed of EPC/EPG (95:5 molratio) (triangles) which is liquid-crystalline at 37° C. and DSPC/DSPG(95:5) (triangles) a gel-state lipid matrix at the body temperature ofthe mouse. Phosphatidylglycerol (PG) was incorporated to impart asurface negative charge, necessary to prevent the gel-state vesiclesfrom aggregating in the absence of cholesterol as described in Nayer etal., Biochim. Biophys. Acta, 986:200-206 (1989), incorporated herein byreference. Reliable comparison of the two systems was facilitated byadding a negative charge to the EPC vesicles. The results, presented inFIG. 6A, reveal that the gel-state vesicles produced a delayed increasein plasma cholesterol which did not peak until after 24 h, whereasEPC/EPG vesicles (circles) gave rise to a cholesterol profile similar tothat observed for EPC alone (FIG. 1). Control is saline (squares).

The data in FIG. 6A demonstrate that the rate of cholesterolaccumulation for these two types of vesicle was the same. The differentplasma cholesterol profiles occurred because approximately 70% of theDSPC/DSPG vesicles (triangles) were cleared within 4 h compared to lessthan 30% of the EPC/EPG LUV₁₀₀ (circles) (FIG. 6B). The bulk ofcholesterol mobilization occurred in the first 24 h, consequently liquidcrystalline EPC/EPG removed more than 3000 nmol to the RES, whereasDSPC/DSPG vesicles removed 1700 nmol. The source of the accumulatedliposomal cholesterol and its fate was determined. Ultimately,cholesterol efflux must occur from atherosclerotic plaque to achieveregression. However, it is known that the cholesterol within cells andatherosclerotic lesions equilibrates more slowly than sterol present inplasma membranes directly exposed to acceptor particles. Movement ofthis cholesterol will be a secondary event initiated by the primaryefflux of outermembrane cholesterol.

In a 20-g mouse approximately 35% of the circulating sterol isassociated with lipoproteins and about 65% with the plasma membranes oferythrocytes. However, all of the sterol associated with erythrocytes isfree cholesterol, whereas a large proportion of lipoprotein sterol isesterified. Consequently, the largest pool of free cholesterol in thecirculation is in the red blood cell plasma membrane. It was found thatthis source of cholesterol does not change significantly in the presenceof liposomes, despite a two fold increase in plasma sterolconcentration. This result is shown in FIG. 7A.

Erythrocyte membrane cholesterol can be depleted by liposomes in vitro.Consequently it was determined whether erythrocytes act as the primarysterol donor and which then are rapidly replenished by lipoproteinswhich are in turn able to extravasate and scavenge more sterol fromperipheral tissues. Erythrocytes were isolated from mice and labelledwith [³H]cholesterol in vitro. The labelled cells were injected into agroup of mice, half of which were subsequently treated with saline andhalf with 300 mg/kg of EPC LUV₁₀₀. The specific activity of red bloodcell cholesterol was determined over an 8-h time-course and the twogroups compared. As demonstrated in FIG. 7B, the decrease in cholesterolspecific activity is the same for both the control and experimentalgroup. Interpretation of these data is limited by the fact that cellslabelled in vitro are also removed from the circulation over a similartime-course (determined by chromium labelling). However, it can beestimated that at least 50% efflux of cell sterol would be necessary toaccount for the rise in plasma cholesterol observed after 8 h. Thiswould result in a considerable dilution of erythrocyte cholesterol ifthis sterol pool were continuously replenished. As this has not beenobserved, the data suggest that red blood cell cholesterol is not theprimary source of the liposomal sterol accumulated in vivo.

C/P ratios of lipoproteins (circles) showed a significant decrease overcontrol values in the first 8 h (FIG. 7C). The ratio returned to normalvalues after 8 h mirroring the time-course of cholesterol accumulationby vesicles. This suggests that it is primarily lipoprotein cholesterolin equilibrium with circulating liposomes, and that lipoproteins mediatethe transfer of cholesterol from peripheral tissues to liposomes. Theresults are also consistent with observations in vitro that indicatecholesterol can undergo desorption from lipoproteins more readily thanfrom erythrocytes. Finally, the rate of cholesterol accumulation byLUV₁₀₀ in vivo (FIG. 3) is considerably faster than that observed invitro (FIG. 5), indicating that the rate of cholesterol desorption fromsources in vivo is greater than from the 100 nm vesicle donors used toobtain the data in FIG. 5.

Example 2 Regression of Atheromas in Rabbits Treated with Liposomes

This example demonstrates mobilization of cholesterol and regression ofatheromas in rabbits treated with liposome compositions of the presentinvention. Plasma cholesterol concentration increased 2.5 times inliposome treated rabbits. Aortic lipid content decreased 25% in liposometreated animals.

Egg phosphatidylcholine (EPC) was supplied by Princeton Lipids(Princeton, N.J.). A 0.5% cholesterol supplemented diet was obtainedfrom Teklad Premier. Blood collection tubes and butterfly needles (23gauge) were from Becton-Dickinson (Missisauga, Ontario). Ketamine,xylazine, heparin, Innovar and Euthanyl were supplied by MTCPharmaceuticals, Janssen Pharmaceutics and Organon Technika (Ontario).Bio-Gel™ A-15m was purchased from Bio-Rad. Prepacked Solid Phase silicagel columns were acquired from Burdick & Jackson. All chemical andsolvents were of analytical grade from BDH Chemicals (Vancouver, B.C.)

Forty eight New Zealand White (NZW) rabbits were housed in wire cages atthe Animal Unit of the Research Centre conforming to guidelines set bythe Canadian Council on Animal Care and the University of BritishColumbia. The animals were maintained in a controlled temperatureenvironment with a 12 hour dark/light cycle. Approximately 150 g of foodwere given per animal per diem. Water was freely given.

Lesions induced in rabbits as a result of maintaining the animals oncholesterol enriched diets for more than two months, do not regress forlengths of up to two years even when they are returned to standardrabbit chow. St. Clair, Prog. Cardiovasc. Dis., 26:109-132 (1983). Evenafter cessation of cholesterol enriched diets, lesions have been notedto progress and increase in complexity. Prior et al., Arch. Path.,71:82-94 (1961). Moreover, in cases where intermittent feeding scheduleswere administered or a low cholesterol-enriched diet was given over aperiod of years, lesions similar to the calcified ulcerated lesionsobserved in humans have been produced. Constantinides et al., Arch.Pathol., 70:81-92 (1961).

The correlation between hypercholesterolemia and the onset andprogression of atherosclerosis in the rabbit is well established. St.Clair, supra. To ensure that an equal distribution of animals weredivided into the respective treatment groups, careful pairing of theanimals was done. Initially, the 48 NZW weanlings were screened forresponders to the 0.5% cholesterol enriched diet (Teklad diet 0533). Theanimals were fed the cholesterol diet for one week and plasmacholesterol concentrations monitored until returning to normal. Animalswere matched by the extent of the rise in plasma cholesterol levels aswell as the rate at which the levels returned to normal. This enabled anequal distribution of animals to be placed into two groups of 24 thatwere fed either standard rabbit chow or 0.5% cholesterol enriched rabbitchow for 20 weeks to induce atherosclerotic plaque formation. Duringthis time, plasma lipid levels were monitored on a monthly basis. Twoanimals were euthanized due to complications probably associated withhandling and were excluded from the final analyses. After the dietinduction period, five animals from each group were sacrificed to verifythe formation of lesions and serve as the standards against which theeffectiveness of liposomal treatment was assessed. Thereafter, allremaining animals were fed regular rabbit chow until the conclusion ofthe study.

Rabbits were fed a 0.5% cholesterol-enriched diet for 20 weeks in orderto induce intermediate lesions more significant than fatty streaksassociated with shorter duration cholesterol-enriched diets. Chemicaland histological analyses of aortas obtained from rabbits following thediet induction period, but prior to treatment, revealed plaques formedthat were rich in lipid and surrounded by fibrous tissue. These plaquesconsisted of almost equivalent amounts of cholesterol and cholesterolester. The aortic phospholipid in these animals was 15±4 μmol/g wettissue and aortic total cholesterol was 114±28 μmol/g wet tissue (61±13μmol/g cholesterol and 53±15 μmol/g cholesterol ester). Animalsmaintained on a standard diet had aortic phospholipid levels of 4±0.3μmol/g wet tissue and aortic total cholesterol levels of 10±1 μmol/gwhich was predominantly cholesterol. The degree of surface plaqueinvolvement in cholesterol fed animals was 78±14%.

Based on the pairing of plasma cholesterol concentrations, 18 rabbitsremaining from each diet group were separated into groups of 9 and weretreated with EPC LUV₁₀₀ at a dose of 300 mg/kg or the equivalent volumeof saline. Treatment was initiated 4 weeks after return to standardrabbit chow and was given over a 100 day period. The treatment consistedof ten bolus injections of phospholipid or saline administered into themarginal ear vein. One injection was given every 10 days.

The rabbits ranged from 4-6 kg in weight. Each treatment of thevesicle-receiving rabbits required the preparation of approximately 150mls of LUV₁₀₀ at a concentration of 200 mg/ml. Typically, 6 gramaliquots of EPC were hydrated with 30 ml of filtered 150 mM NaCl, 20 mMHEPES (HBS), pH 7.4, in sterile 50 ml conical tubes, vortexed and keptovernight. As described in Example 1 above,, the resulting multilamellarvesicles (MLVs) were used to generate LUV₁₀₀ by extrusion through twostacked polycarbonate filters of 100 nm pore size using a 10 mlwater-jacketed thermobarrel Extruder (Lipex Biomembranes, Vancouver,B.C.), according to the method of Hope et al., Biochim Biophys. Acta,812:55-65 (1985), incorporated herein by reference. Vesicle sizes weredetermined by quasi-electric light scattering (QELS) analyses utilizinga Nicomp Model 370 submicron laser particle sizer (Pacific Scientific,MD). The vesicles used for the 10 treatments had an average diameter of114±7 nm.

A small dose of Innovar™ was given to promote calmness and vesseldilation in animals to ease routine bleedings necessary for plasma lipidanalyses. To facilitate the final blood collections, ketamine (40 mg/kg)and xylazine (8 mg/kg) were given intramuscularly to sedate the animals.Fifty units of heparin (Hepalean) followed by a lethal dose ofphenobarbital (Euthanyl) were then perfused into the marginal ear veinbefore laparotomy. Organs were removed, rinsed in saline and immediatelyfrozen in liquid nitrogen. The heart and full length aorta werecollected in one section and kept in iced saline. The animals weresacrificed in groups of 8-10 on alternate days. The organs wererandomized prior to processing and analyses.

Each aorta was separated from the heart at the aortic valve and wascarefully cleaned to remove any adherent adventitial fat. The aortaswere cut along the ventral surface, opened, and photographed on a blackbackground. The photographs were used in conjunction with the negativesto aid in the collection of digitization data as well as to facilitatethe division of the aortas into three regions: the arch, thoracic, andabdominal aortic segments as described by Rosenfeld et al.,Atherosclerosis, 8:338-347 (1988), incorporated herein by reference.Nine animals were in each of the 4 treatment groups: (1) vesicle-treatedcholesterol-fed animals (VC), (2) saline-treated cholesterol-fed animals(SC), (3) vesicle-treated normal diet animals (VN) and (4)saline-treated normal diet (SN). Six aortas from each group wereallocated for lipid analyses and stored at −20° C. until analysis. Theremaining three samples in each group were fixed in 10% neutral bufferedformalin for at least 48 hours and used for gross staining with Sudan IVand histology. Holman et al., Lab. Invest., 7:42-47 (1958), incorporatedherein by reference. At the time of lipid analysis, the aortas werepatted dry and divided into the three segments. Wet weight and lengthwere measured and the aortic segments were homogenized (Polytron) inHBS. Two additional washes of the Polytron™ probe with HBS werecollected for each segment to ensure complete homogenate recovery.

Whole aortic segments were analyzed by digitization. In this analysis,photographic negatives obtained from all unstained aortas wereilluminated generating an image using a Microcomputer Imaging Device(Imaging Systems). The percentage of plaque involvement was calculatedby dividing the area occupied by surface plaque by the area of theentire aorta segment. Distinct differences were observed in the degreeof shading of plaques and uninvolved aortic tissue. Assessments of thepercentage of atherosclerotic plaque involvement were performed by twoobservers and the results were averaged. Interobserver variation waswithin ±5%.

Cholesterol and phospholipid content of the aortas and livers of thesacrificed animals were quantified following Bligh and Dyer extractionsof the homogenates. Bligh and Dyer, Can. J. Biochem. Physiol.,37:911-917 (1959), incorporated herein by reference. Total cholesterol,cholesterol, and cholesterol ester contents were determined according tothe method of Rudel and Morris, J. Lipid Res., 14:364-366 (1973),incorporated herein by reference. Cholesterol and cholesterol esterswere separated by silica gel chromatography on Burdick and Jacksonprepacked 200 mg Solid Phase Silica Gel columns. Cholesterol esters wereeluted with 1 ml methylene chloride. Cholesterol was collected followingmethylene chloride/methanol (95:5) elution after transferring thecolumns to a new carrier. Phospholipid content was measured according toFiske and Subbarow, J. Biol.

Chem., 66:375-400 (1924), incorporated herein by reference.

Lipoprotein lipid profiles were quantified by enzymatic procedures afterphosphotungstic acid precipitation.

Aliquots of aorta or liver homogenates were incubated overnight at 37°C. with 1 ml of 1N NaOH. Thereafter, sodium dodecylsulphate (SDS) wasadded to the mixture to make a 1% solution needed to solubilize anyremaining particulate matter. Protein content of the samples wasquantified by the bicinchoninic acid (BCA) protein assay method (PierceChemical Company, Rockford, Ill.) after incubation for 1 hour at 60° C.and read at A₅₆₂ against an albumin standard.

Typically, 2-3 mm segments from the arch, thoracic, and abdominal aortaof three different animals within each treatment group were divided intoleft and right halves and embedded in paraffin. At least 8 segments fromeach region were prepared as blocks, depending on the length of theaorta. Alternate sections of 5 μm were adhered to gelatin coated slidesfrom paraffin blocks and visualized with hematoxylin and eosin (H&E) orWeigart's-van Gieson's stains. Intima/media ratios of the differentregions were calculated by initially measuring an average ratio from 3photographs generated from each section and using this value todetermine a final mean±standard deviation from all the sections madefrom the animals of each group.

The nature of plaques from animals sacrificed after the diet inductionperiod, but prior to any treatment was examined after sections were madefrom segments held into place with tissue mount (OCT) on wooden stagesand quick frozen in isopentane followed by liquid nitrogen.Subsequently, alternate sections of 5 μm were adhered to polylysinecoated slides and visualized with Sudan IV differentiated with Harris'hematoxylin, H&E or van Gieson's stains to highlight lipids andcollagen.

Unless otherwise indicated, mean±standard deviation values arepresented. The significance of the difference of the means was assessedby an analysis of variance using the two-sample t test. Only values ofP<0.05 were considered significant.

During the course of this study, animals maintained on theatherosclerotic diet exhibited plasma total cholesterol concentrationsranging from 5-10 times that of the control animals fed the standarddiet while fed the cholesterol-enriched diet. The cholesterolconcentrations remained elevated (2-5 times higher) until the conclusionof the study even though standard rabbit chow was given during thetreatment period. This is illustrated in a typical time course ofcholesterol mobilization resulting from the infusion of 300 mg/kg EPCLUV₁₀₀ or an equivalent volume of saline demonstrated in FIG. 8. Acomparison of control animals injected with saline demonstrates thatanimals previously fed the high cholesterol diet (panel A) maintainedplasma cholesterol concentrations 3 times higher than animals maintainedon the standard diet throughout the study (panel B) even though thecholesterol diet was terminated 10 weeks earlier. Despite theatherosclerotic animals having excess plasma cholesterol, an injectionof LUV₁₀₀ resulted in a dramatic 2.5 times increase in plasmacholesterol concentrations in both hyper- and normocholesterolemicanimals when compared to saline treated counterparts. Plasma cholesterollevels peaked at 24 hours post-infusion before returning to baselinelevels after 5 days. This time course correlates with the removal ofvesicles from the circulation measured as total plasma phospholipidconcentration illustrated in the clearance profiles shown in FIG. 9.Although atherosclerotic animals had slightly higher total phospholipidconcentrations, similar clearance kinetics of the injected vesicles wereseen between normal and hypercholesterolemic rabbits.

As demonstrated in Example 1 above, the amount of cholesterolaccumulated and removed by liposomes with each infusion is a function ofthe rate of liposomal cholesterol uptake and the rate of vesicleclearance. Also, it was determined that all cholesterol above salinetreated levels was associated with circulating liposomes by generating acholesterol and phospholipid profile after separating vesicles fromplasma by gel filtration. This showed that excess plasma cholesterol wasassociated with the vesicles and that >90% of the cholesterol was freecholesterol. Hence, an estimate of the mass of cholesterol removed fromthe circulation (mostly by the RES) was made by calculating the C:Pratios of vesicles at intervals following each injection from plasmaphospholipid concentrations (vesicle-treated concentration minussaline-treated concentrations) and cholesterol (excess plasmaconcentration above the control concentration) at different time pointsduring the experiments.

The plasma volume of the rabbits was approximately 150 ml. An estimateof the cholesterol removed was calculated employing the average C:Pratio measured for vesicles at each assay interval. This data is shownin FIG. 10. The data represents an average standard deviation expressedas mmol of cholesterol removed with each treatment inhypercholesterolemic animals and was calculated from data obtained fromtreatments 1, 4 and 10. The analysis was not continued beyond the pointwhere less than 10% of the initial phospholipid dose remained in thecirculation. Below this level, the measurement error was too large todetermine accurate C:P ratios. After 104 hours it was estimated thatapproximately 1 mmol of cholesterol was removed from the circulation bythe RES, which represents approximately 50 mole % of the injectedphospholipid dose. Furthermore, based on plasma cholesterolconcentrations measured in animals 24 h post-injection, each of the 10infusions of liposomes caused dramatic cholesterol mobilization.

The ability of the animals to tolerate and remove repeated injections ofphospholipid and the consequences of administering excess phospholipidon plasma lipid levels were examined. Chronic short term (one week)administration of Intralipid, an emulsion of triglycerides andphospholipids, causes increased LDL levels. Although the phospholipidcontent of Intralipid is comparable to the dose of 300 mg/kg LUV₁₀₀ perinjection of the present treatment regimen, Intralipid is generallygiven intravenously on a daily basis as a nutritional supplement.

Each injection of 300 mg/kg EPC LUV₁₀₀ apparently induces a transient100-fold increase in plasma phospholipid concentrations and at the endof liposomal therapy (10 injections) each animal received an averagetotal dose of 12-20 mmol (10-15g) of phospholipid. The clearanceprofiles of several injections of EPC LUV₁₀₀ in cholesterol fed rabbitsis shown in FIG. 11A. As illustrated, significant differences in therates of vesicle clearance between injections were not detected. FIG.11B shows that similar concentrations of vesicle phospholipid remain inthe circulation 24 h post-injection in both normo- andhypercholesterolemic animals following serial injections. If the abilityof the fixed macrophages of the RES were compromised, increasingphospholipid levels would likely be detected during the latertreatments. Furthermore, 5 days post-injection, the injected dose ofliposome phospholipid was completely removed from the circulation andplasma phospholipid and cholesterol concentrations returned to baselinelevels.

At the conclusion of the study, saline-treated cholesterol-fed animalsmaintained elevated plasma cholesterol levels whereas vesicle-treatedanimals had levels comparable to animals maintained on the standarddiet. The reduction in plasma cholesterol concentrations ofvesicle-treated atherosclerotic animals resulted from a reduction inboth plasma LDL and HDL cholesterol concentrations although the relativeproportions of HDL/LDL cholesterol were not affected. No changes in theplasma lipid profiles (cholesterol, phospholipid or triglycerides) weredetected in animals maintained on standard rabbit chow throughout thestudy. Plasma phospholipid levels in vesicle-treated animals weresimilar to their saline-treated counterparts despite the injection ofapproximately 15 grams of phosphatidylcholine per animal duringliposomal therapy. These results, unlike those observed with Intralipidinfusions, suggest that repeated administration of LUV₁₀₀ given at 10day intervals does not compromise RES function or normal plasma lipidhomeostasis.

Erythrocyte cholesterol remained constant throughout the infusions.However, a decrease in the C:P ratios of lipoproteins was detected overthe first 24 hours. This C:P reduction gradually returned to normallevels after 48 hours (see FIG. 12). This time course mirrorscholesterol accumulation by the vesicles. These results suggest that thelipoprotein pool of cholesterol rapidly equilibrates with the vesiclesand supports the hypothesis that liposomes generate cholesterol-poorlipoprotein particles that can access peripheral tissues and promotecellular cholesterol efflux.

The extent of lesion progression or regression was assessed by threecomplementary methods: (1) chemical lipid and protein assays todetermine lesion bulk, (2) digitization of gross surface morphology toquantitate the degree of plaque involvement, and (3) histochemistry toexamine the nature and depth of the lesions.

Despite elevated plasma cholesterol concentrations persisting in animalsreturned to standard rabbit chow, saline-treated animals were found tohave arterial wall cholesterol content expressed per gram wet weight of94±12 μmol/g total cholesterol, 58±6 μmol/g free cholesterol and 37±9μmol/g cholesterol esters with an average surface plaque involvement of77±17%. Although there appears to be slight reduction in the cholesterolester content, the values of the lipid content of saline-treated animalswere not significantly different from values found in atheroscleroticanimals prior to treatment indicating that there was no progression orregression of lesions after 4 months. On the other hand liposome-treatedanimals were found to have significantly less cholesterol content of theentire aorta with levels of 85±8 μmol/g total cholesterol, 48±5 μmol/gfree cholesterol and 37±6 μmol/g cholesterol esters. Because there wereno significant differences between the lipid content of animals beforeor after saline treatment, the reductions in plaque cholesterol contentbetween liposome- and saline-treated animals indicates regression, notsimply decreased progression, of plaques.

Aortic lipid content was expressed per gram of protein weight as wetweights are likely to be more variable. No significant differences werefound between the protein levels in both saline- and vesicle-treatedanimals. The protein content of the aortas to be 0.41 g protein/g wetweight and 0.43 g protein/g wet weight, respectively. Expressing thedata per g protein, liposomal therapy resulted in a 25% reduction intotal cholesterol content of the entire aorta of vesicle-treated animalscompared to saline-treated controls. By segment, there was a 48%reduction seen in thoracic aorta cholesterol levels and small reductionsin the arch (AR) and abdominal (AB) aortas (see FIG. 13A). Significantreductions in the cholesterol ester levels in vesicle-treated animalswere also noted and again the thoracic™ aorta demonstrated the greatestdecrease (see FIG. 13B). In addition to decreased cholesterol content,aortic phospholipid levels in vesicle-treated atherosclerotic animalsdecreased, although not to the level of statistical significance.

In order to maximize the number of animals within each group, allnegatives generated from photographed unstained aortas (AR) weredigitized. Gross Sudan IV staining of 3 aortas from each treatment groupconfirmed the same degree of surface plaque involvement as unstainedaortas. The area of plaque involvement was determined by digitization.The data is shown in FIG. 13C. Liposome-treated, cholesterol fed rabbitsdemonstrated 61±13% involvement of the entire aorta compared to 77±17%involvement of saline-treated animals, representing an overall 16%reduction of surface plaque. In agreement with the reductions incholesterol content detected by lipid analyses, the thoracic aortaexhibited the most benefit from liposome infusion with digitizationanalysis and displayed a 26% reduction in plaque involvement, whereasthe abdominal aorta revealed a 16% reduction. There was a slightreduction in the degree of surface plaque involvement of the arch thatfailed to reach statistical significance. No significant differencesbetween treated and untreated control animals maintained on the standarddiet were seen and both groups showed essentially no plaque involvement.

Histochemical analysis revealed extensive raised plaques (intimalthickening) in the cholesterol fed animals as expected from grosssurface morphology inspection. Whereas digitization quantitated theextent of plaque involvement, histochemical analyses allows the depthand nature of the lesions to be assessed. Generally, the plaquesexhibited extensive intimal thickening due to stratified lipid depositsthat were surrounded by a collagenous network. The arch region was notedto display more advanced lesions of apparent crystalline cholesteroldeposits and showed a few isolated necrotic foci as detected with H&Estaining.

Representative sections of the thoracic aorta of vesicle-treated andsaline treated animals revealed that lesions of animals treated withvesicles manifested fewer lipid deposits and showed moderately reducedplaque thickening when compared to saline treated atheroscleroticanimals. This is quantified in Table 1 summarizing the data obtainedfrom the analysis of pictures taken from multiple sections used toassess the severity of lesions present in the arch, thoracic orabdominal aorta of atherosclerotic animals. As can be seen, a decreasein the intima/medial ratios in the arch and thoracic regions of liposometreated animals were detected, whereas no changes were detected in theabdominal aorta. No apparent differences were detected between treatedand untreated animals maintained on the standard diet throughout thestudy.

Cholesterol feeding of rabbits often leads to the accumulation ofcholesterol in a number of tissues including the liver. However upon thereturn to regular rabbit chow, non-arterial tissue cholesterol levelsoften revert to normal within a month. Liver cholesterol content wasmeasured in order to gain insight into whether (1) increased biliaryexcretion of cholesterol might be occurring in liposome-treated animalsdue to massive deposition of the injected phospholipids in the liverresulting in reduced liver cholesterol levels or (2) there was adetrimental accumulation of cholesterol mobilized by the liposomes tothe liver. In atherosclerotic animals, liposome-treated rabbitsdemonstrated a slight reduction in liver cholesterol content havingaverage levels of 8 μmol/g that are comparable to control animals fedthe standard diet. Saline-treated animals exhibited average levels of 11μmol/g. This difference was not statistically significant.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated by reference into thespecification to the same extent as if each individual publication,patent or patent application was specifically and individually indicatedto be incorporated herein by reference.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims. Table 1.Measurement of Intima/Medial ratios in the different regions of theaorta of vesicle and saline treated atherosclerotic animals.

Intimal/Medial Ratios Significance Portion of aorta Liposome TreatedSaline Treated (P value) Arch 1.51 ± 0.55 1.76 ± 0.94 N.S. Thoracic 1.34± 0.73 1.93 ± 1.12 P < 0.01 Abdominal 1.84 ± 0.95 1.81 ± 1.25 N.S.

What is claimed is:
 1. A method for treating atherosclerosis comprisingparenterally administering to a subject in need of such treatment, apharmaceutically effective amount of a homogeneous population of largeunilamellar liposomes consisting essentially of phospholipid which is ina liquid crystalline phase at 37° C., wherein the population isobtainable by extrusion of multilamellar vesicles consisting essentiallyof the phospholipid through two stacked polycarbonate filters having apore size of 0.1 micron, wherein the population of liposomes has anaverage diameter between about 100 and 150 nm.
 2. The method of claim 1,wherein the population has a mean diameter of 125±30 nm.
 3. The methodof claim 1, wherein the population has a mean diameter of 114±7 nm. 4.The pharmaceutical composition of claim 1, wherein the population has anaverage diameter of about 125-140 nm.
 5. The method of claim 1, whereinthe phospholipid is selected from the group consisting ofdistearoylphosphatidylcholine, dipalmitoylphosphatidylcholine, oleoylphosphatidylcholine, palmitoyl-oleoyl phosphatidylcholine, dioleoylphosphatidylcholine, sphingomyelin, phsophatidylglycerol, and mixturesthereof.
 6. The method of claim 1, wherein the phospholipid isphosphatidylcholine.
 7. The method of claim 1, wherein the liposomes arebound to a protein or peptide.
 8. The method of claim 1, wherein thepopulation is administered in an amount of from about 0.1-1.5 g ofliposomes per kg of body weight.
 9. The method of claim 1, wherein thepopulation is administered in an amount of from about 0.2-0.75 g ofliposomes per kg of body weight.
 10. The method of claim 1, wherein thepopulation is administered in an amount of from about 0.28-0.42 g ofliposomes per kg of body weight.
 11. The method of claim 1, wherein thepopulation is administered intravenously.
 12. The method of claim 1,wherein the population is administered once a week over 4-16 weeks. 13.The method of claim 2, wherein the population is administered once aweek over 10 weeks.
 14. The method of claim 1, wherein serum is obtainedfrom the subject and measurements of at least one of total freecholesterol, total esterified cholesterol, HDL cholesterol, LDLcholesterol and VDL cholesterol are carried out during the treatment.15. The method of claim 1, wherein the subject is afflicted with atleast one atherosclerotic plaque, and the method results in reduction ofcholesterol content in the plaque or reduction in volume of theathelersclerotic plaque.
 16. The method of claim 1, wherein the subjectis afflicted with at least one atherosclerotic plaque, and the methodresults in the inhibition or prevention of formation or expansion of anatherosclerotic plaque.
 17. The method of claim 1, wherein the subjectis afflicted with at least one atherosclerotic plaque associated withfamilial hyperlipidemias.
 18. The method of claim 1, wherein the subjectis afflicted with atherosclerosis-associated congestive heart failure.19. The method of claim 1, wherein the subject is afflicted withatherosclerosis-associated severe hypertension.
 20. The method of claim1, wherein the subject is afflicted with at least one atheroscleroticplaque associated with hypoalphalipoproteinemia.
 21. The method of claim1, wherein the subject is afflicted with at least one atheroscleroticplaque associated with hypercholesterolemia.
 22. The method of claim 1,wherein the atherosclerosis is found in a blood vessel of the subjectselected from the group consisting of an aorta, a carotid artery, acoronary artery, a renal artery, an iliac artery and a popliteal artery.23. The method of claim 1, wherein the vascular regions of the subjectshave injured epithelium.
 24. The method of claim 23, wherein the injuredepithelium is due to coronary angioplasty or vascular bypass grafting.25. The method of claim 24, wherein the coronary angioplasty is due totreatment of restenosis of lesions in coronary arteries.
 26. The methodof claim 1, wherein the population is administered along with apharmaceutically acceptable carrier.
 27. The method of claim 1 whereinthe population is administered to a patient to suppress a rise in plasmaconcentration of atherogenic lipoproteins.
 28. The method of claim 27wherein the rise in plasma concentration is due to a genetic cause.